Difference between Sequence Diagram (SD) and a System Sequence Diagram (SSD)?

Question

I'm working on a project for a grad class and still having trouble wrapping my head around them.

What is the difference between a sequence diagram (SD) and a system sequence diagram (SSD)?

And in what order should they be developed when working on a systems development project?

Answer 1

A System sequence diagram visualizes a use case, while a sequence diagram visualizes a method of a class.

The elements participating (exchanging messages) in a system sequence diagram are Actors and Systems. The messages exchanged by these elements could be any type depending on the systems (from web service calls to data input from a human).

The elements participating in a sequence diagram are objects (instances of various classes). The messages exchanged by these elements are method invocations.

First you would analyze and document the use cases. Each use case describes a main business task required. The system use case diagram is part of this activity.

Later you come to design each subsystem, namely each element participating in the system use case diagram. You draw one or more class diagrams depicting the classes participating to the solution. Then you would draw a sequence diagram for the most important (or difficult to understand) methods.

EDIT: Some examples can be found in the agile modelling site.

Answer 2

Sequence Diagram: It shows interaction between two objects. System Sequence Diagram: It shows interaction between system & actors.